Card edge connectors are well-known for electrically connecting circuit traces on a daughterboard with mating circuit traces on a motherboard. A daughterboard generally comprises a circuit board having a plurality of electronic devices mounted thereon and having electrical circuitry which terminates in contact pads disposed on spaced apart centers along an edge portion of the board. The card edge connectors generally comprise a housing which defines a slot dimensioned to receive the edge portion of the circuit board, and terminals having one or more contact portions which extend into the slot for engagement with the contact pads of the circuit board. The terminals are disposed on spaced apart centerlines corresponding to the spacing of the contacts along the edge of the circuit board. Such a connector is disclosed in U.S. Pat. No. 4,557,548 wherein each of the terminals has a pair of spring contact arms extending into the slot, the arms being electrically joined to a common lead.
It is known to provide a card edge connector with multiple terminals at each terminal centerline for engagement with respective contact pads which are disposed on opposite sides of the board at different distances from the board edge. See, for example, U.S. Pat. Nos. 4,298,237 and 5,024,609. These patents disclose high-density connectors wherein multiple terminals on a common centerline have spring arms which extend to different levels above a bottom of the slot for engaging respective contact pads which are disposed at different distances from the insertion edge of the circuit board. In each of these connectors, the daughterboard in inserted into the slot with a straight line motion, and all of the terminals engage their respective contact pads simultaneously as the daughterboard is fully seated in the connector.
A problem with high-density, direct insertion connectors is that the sum of the forces exerted by the individual spring arms is quite large, thereby resulting in considerable resistance to insertion or removal of the daughterboard. In order to overcome this problem, card edge connectors requiring a zero or low insertion force have been developed. See, for example, U.S. Pat. Nos. 3,795,888; 3,920,303; 4,185,882; and 4,575,172. Each of these patents discloses a connector wherein a circuit board can be inserted into the connector at a first angular position against little or no resistance, and the circuit board can be pivoted to a second angular position wherein the terminals are deflected in opposition to their spring force. Pivoting insertion substantially reduces friction of the terminals against their contacts as compared to straight line, or direct, insertion, and provides a mechanical advantage which enables easier deflection of the spring contact arms.
The low insertion force card edge connectors have been limited as to the number of terminals which they could accommodate on an individual terminal centerline. U.S. Pat. No. 4,946,403 discloses a low insertion force circuit panel socket wherein individual terminals are spaced on separate centers and each terminal has a pair of contact springs which engage opposite sides of the circuit board.
Another problem with high-density connectors is that closely spaced terminals are more susceptible to electromagnetic induction and cross-talk. Also, the high density connectors can transmit signals at a high rate of speed, but the faster signals generate greater spikes of electromagnetic impulse which are more likely to generate cross-talk between adjacent or proximate terminals.
The connector terminals may connect with either power, signal, or ground circuitry on the daughterboard. In order to prevent stray electromagnetic impulses from damaging electronic components on the daughterboard, it would be advantageous for the power and ground circuits to be complete before the signal terminals are electrically connected. This would give stray impulse spikes from the signal circuitry a straight path to electrical ground. As previously discussed, the connectors disclosed in the '237 and '609 patents have terminals which engage all of their respective contact pads simultaneously as a daughterboard is inserted therein.
The present invention provides a low insertion force card edge connector, or socket, having multiple terminals on each terminal centerline. The terminals are arranged such that some of the terminals will engage their respective contacts on the daughterboard before others of the terminals, thereby providing a connector with make first, break last terminal connections.